The role of the human basophil in health and disease is not fully understood. Our laboratory has developed an in vitro model for studying basophil function. Two unique physiologic stimuli have been identified: 1) a fragment from the fifth component of complement, and 2) a factor generated by human lymphocytes when incubated with appropriate antigens or mitogens. We have recently observed that cells obtained by thoracic duct drainage of individuals awaiting renal allografting will also produce this second factor. This has provided sufficient quantities for initial characterization. The histamine releasing factor made by lymphocytes has a molecular weight of 12,000 daltons and seems to have two separate components when separated by electrophoresis. This material will be used for further molecular characterization of the factor and comparison with other known lymphokines. The guinea pig is the only laboratory animal with basophils which function in a manner reasonably similar to man. We have recently described antigen-induced release of histamine from guinea pig basophils and have studied this over four months in a single animal. We have also passively transferred sensitivity with serum. This model will be used to further define the importance of various cell types, immunoglobulin classes, and methods of immunization in regulating the function of basophils.